Residual monomers and oligomers tend to remain in the reaction mixture following polymerization of monomers to form polymers. Many different approaches are taken to remove them, ranging from "chasing" the reaction mixture with additional polymerization initiator to passing the mixture through an ultrafiltration membrane whose molecular-weight cutoff is higher than the molecular weight of the oligomers but substantially lower than that of the polymer. Ion exchange has also been used to remove monomers and oligomers from polymers.
Residual monomers and oligomers may have varying degrees of toxicity when the polymer containing them is administered to the body, intravenously, orally or otherwise. Because of this, it is particularly desirable to reduce residual monomers and oligomers to low levels in polymers intended for pharmaceutical applications; preferred is less than 1000 parts per million (ppm) and more preferred is less than 100 ppm.
Of particular interest are polymers that are useful for controlling cholesterol in the blood of animals or human beings. Strongly basic, polymeric ion-exchange materials form an ionic bond with the acid groups of bile acids, removing those acids from the digestive tract before they can form cholesterol. Reducing the levels of residual monomers and oligomers in these polymeric materials is particularly important to their pharmaceutical acceptability.